1. Field of the Invention
The present invention relates generally to systems for transferring liquid from a storage tank to an output line, and more particularly to liquid transfer systems utilized in the semiconductor industry wherein high purity and accurate liquid flow rate controls are necessary.
2. Description of the Prior Art
Many semiconductor manufacturing steps require the utilization of high purity liquids, such as acids and solvents, and various liquid transfer systems are utilized to deliver such liquids. Because the manufacturing steps must be precisely controlled, similar precise controls are required in the delivery of the liquids, such as precise liquid flow rates with minimal variance in the flow rates. Therefore, liquid delivery systems which rely upon pumps to move the liquid are less desirable than systems which move liquid without utilizing pumps.
Prior art liquid delivery systems that do not use pumps, generally utilize controlled gas pressure within a pressurizable liquid holding vessel to push liquid from the vessel in a controllable manner. Systems utilizing two pressurizable liquid holding vessels to alternatively deliver liquid are known in the prior art, wherein one vessel is filled while the second vessel is pressurized to dispense liquid therefrom. By alternately filling and dispensing liquid from two pressurizable vessels, a constant, controllable flow of liquid is obtained. However, the inputting of liquid into the vessels during the fill cycle can be problematical. The prior art utilizes a constant vacuum, pumping, or significant pressure to move liquid from a supply tank to each vessel during the fill cycle. The constant vacuum or pressure can alter the delicate chemistry of some types of liquids, such as by removing volatile organic compounds from solvents or adding small bubbles into the liquids, where such bubbles are detected and identified as particulate impurities in the liquid. Pumping such high purity chemistries can contaminate with both particles and trace metal ionics.
The present invention provides an improvement on such liquid transfer systems by placing the pressurizable vessels beneath the supply tank, such that a siphon effect can be utilized to transfer liquid from the supply tank to the vessels. Once a siphon effect has been established, there is no further need for pressure or continued vacuum effect to move the liquid from the supply tank to the vessels, thus improving the quality of the output liquid from the system.